In the Chaplet we petition the Father, through Christ's Passion, to have mercy upon us and the world. We can focus our petitions for mercy upon specific people and intentions, as our Lord taught St. Faustina to do in the Novena to Divine Mercy. I will sometimes picture a different person's face for each of the fifty times I pray, "For the sake of His sorrowful Passion, have mercy upon us and on the whole world" - family members, friends, clergy, government leaders.
The majority of the time, however, I try to focus my mind upon what calls forth the Father's mercy, our Lord's Passion. I will focus each decade upon a different one of our Lord's wounds - His back, crown of thorns, hands, feet, side. I sometimes meditate upon the Sorrowful Mysteries of the Rosary. I don't use images very often in my prayer, but one of the young adults I spoke with shared how she will gaze at a crucifix while praying the Chaplet.
There really are a variety of ways to maintain our mental focus while praying the Chaplet. The next time our minds wander during its five decades let's resolve to implement one or more of these methods.